Abstract
This paper discusses the altitudinal boundary between periglacial and non-periglacial zones in northeastern Japan during the Last Glacial Age based on the lower limit of fossil periglacial slopes and the upper limit of distribution of Pleistocene tephra layers.
The characteristics of fossil periglacial slopes in Japan are summarized as follows:
Form: Convex-concave smooth slope on main ridges.
Lower limit of the distribution: ca. 2, 000 m a. s. l. in Central Japan (Table 1). and with height gradually reduced northward.
Slope material: Debris layers or block layers. At present, most of them are stable and covered with alpine and subalpine vegetation.
The lower limit of fossil periglacial slopes is bounded by the upper limit of dissected slopes. The boundary ranges from 100 m to 500 m in height in the upper reaches of the Tama River (Fig. 1). In this area, periglacial block streams in the past and debris flow deposited by heavy rainfall in the present are sporadically distributed around the lower limits of fossil periglacial slopes (Fig. 2). Based on the above-mentioned data, altitudinal morphgenetic zones in the upper reaches of the Tama River during the Last Glacial Age can be divided into four zones as follows: the continuous periglacial slope zone, the sporadic periglacial slope zone, the dissected slope zone, and the stable slope zone. The boundaries of these morphogenetic zones range in height between 2, 000 and 1, 200 m a. s. l. (Fig. 3). From this it may be inferred that the boundary between periglacial and nonperiglacial zones largely varied in altitudinal extent during the Last Glacial Age.
The existence of tephra layers on the mountain slopes and indicates that the slopes have been stabilized since the tephra was deposited. Figure 4 shows altitudinal distribution of marker-tephra layers (1st group: early-middle stage in the Last Glacial; 2nd group: late stage in the Last Glacial) in northeastern Japan. The upper boundaries of both the 1st and 2nd groups are at gradually lower altitudes from south to north. The upper boundary of the distribution of tephra layers rose from the early-middle Last Glacial (1st group) to the late Last Glacial (2nd group). The lower li mits of fossil periglacial slopes in northeastern Japan indicated in previous studies are located above the upper boundary of the distribution of the 1st group. On the other hand, the locations of the macrofossil plants and fossil pollen during the Last Glacial Age were below the upper boundary of the distribution of the 2nd group. These facts indicate the existence of stable slopes covered with vegetation during the Last Glacial Age.
